The main disadvantage of the present method, and of dis- tillation procedures in general, is the difficulty of introducing

irradiated biological samples this is a drawback, owing to the high radiation levels, among other things. Some improve- assistance, ments on the present method along this line might include the fast destruction of organic matter with NalOz fusion and subsequent distillation of bromides from acid solution with a continuous stream of dry HBr gas.

ACKNOWLEDGMENT

The author is greatly indebted to Erik Haeffner, head of the

to Sigrid Hackbarth and Solveig Hellman for skillful technical

automatized operations. In the serial analysis of strongly Chemistry Department, for his active interest in this work and

RECEIVED for review December 5 , 1966. 1967.

Accepted May 4,

Modification of the Fluoride Activity Electrode for Microchemical Analysis

Richard A. Durst and John K. Taylor Institute fbr Marerials Research, Dicision of Analytical Chemistry, National Bureau of Standards, Washington, D. C. 20234

THE FLUROIDE ELECTRODE ( I ) consists of a laser-type single- crystal membrane (europium-doped lanthanum fluoride) which responds to fluoride activity and is sealed into the end of a rigid polyvinyl chloride tube. Inside this tube and in contact with the inner surface of the membrane is a NaF-KCl solution. A silver/silver chloride wire serves as the inner reference electrode and lead wire contact. In normal opera- tion, using a saturated calomel reference electrode, measure- ments can be obtained on as little as one or two milliliters of sample solution. Because the bulky electrode tube is the major limitation on sample size by preventing the immersion of the crystal membrane in smaller volumes, the electrode must be modified for use with sample volumes of less than a mil- liliter.

Operation of the electrode in the inverted position makes possible the use of the membrane itself as the sample “con- tainer’’ and permits measurements on volumes as small as one drop. However, on inversion of the electrode, the en- trapment of an air bubble next to the inner surface of the membrane may cause erratic operation, or complete failure of the electrode if. the solution-membrane contact is broken. To alleviate this problem, it is possible to completely fill the inner compartment of the electrode with the reference solution to eliminate the dead air space. However, the inner construction makes complete filling of the electrode difficult, so that an alternate technique is preferred. This consists of filling the electrode with a gel composed of 4z agar, O.1M NaF, and 0.1M KCI. To accomplish this, the electrode compartment is opened (screw top and inner ring seal) and heated by placing it in a beaker of water on a steam bath. The hot agar/salt solution is then injected into the small inner compartment of the electrode by means of a syringe. The electrode is reassembled and allowed to cool slowly in the normal vertical position. By using this tech- nique, complete filling of the inner compartment is not neces- sary since any air remaining above the gel will not cause im- proper behavior on inversion of the electrode because air bubbles cannot rise through the gel to block the membrane.

After the electrode has been modified for use in the inverted position, it is necessary to confine the sample solution to a

(1) Orion Model 94-09 Fluoride Activity Electrode, Orion Re- search Inc., Cambridge, Mass. [M. s. Frant and J. W. Ross, Jr., Scierice, 154, 1553 (1966)l.

S.C.E.(FIBER JUNCTION1

SINGLE- CRYSTAL FLUORIDE-SPECIFIC MEMBRANE

,-SAMPLE SOLi;TlON(5$pl)

TYGON SLEEVE

MODIFIED FLUORIDE ACTIVITY

ELECTRODE

L lOmm--(

Figure 1. SCE

Modified fluoride electrode microcell and

fixed area of the membrane and to prevent the spreading of a solution film, where the fluoride activity may be different from that in the bulk solution, to other parts of the fluoride sen- sitive membrane. This is accomplished, as shown in Figure 1, by the use of a Tygon tubing (plasticized PVC, 6-mm id . ) sleeve which forms a very tight seal with the raised portion of the membrane. To prevent possible solution entrapment and carryover of sample solutions which would reduce the electrode response, a hydrophobic barrier in the form of a film of silicone lubricant is applied between the crystal membrane and the sleeve.

To complete the cell, the test solution is pipetted onto the membrane, and a fiber junction saturated calomel electrode is lowered into contact with the solution surface (Figure 1). Although 50 pl were used in evaluating the response of this modified electrode, solution volumes as small as 25 p1 are

VOL. 39, NO. 12, OCTOBER 1967 1483

Table I. Summary of the Modified Fluoride Electrode Response Characteristics Fluoride

10-1 95 7 . 0 X

Concn. ( M ) Pgn Activity

10-2 9 . 5 7 . 6 x 10-3 10-3 0.95 7 .7 x 10-4 10-4 0.095 7 .7 x 10-5 10-5 0.0095 7 . 7 x 10-6

5 x 10-6 0.00475 3,85 x 10-6 10-6 0.00095 7 . 7 x 10-7

a Sample vol. = 50 PI. b Ionic strength = 1/zZ,Z,2C, (NaF + KNOB).

Precision expressed as 95 confidence limits (N = 7).

Ionic strengthb

0.200 0.110 0.101 0.100 0 . 100 0.100 0 . 100

Potentia1,c mV C.T. SCE 25.81 i 0 .44 83.43 i 0.47

142.71 i 0.44 202.19 e 0 . 8 1 255.23 i 1 . 3 1 268.30 i 2.45 284.79 * 3.76

70 2.1 2 . 0 1 .9 3 . 5 7 . 1

17.2 106

Precision fig

2 . 0 0.19 0.018 0.0033 0.00067 0.00082 0.00 10 1

sufficient to completely cover the membrane and make con- tact with the reference electrode.

In principle, the electrode responds to fluoride activity, [F-1, according to the Nernst equation:

where E is the measured potential of the electrode, E' is a fixed portion of the electrode potential dependent on the particular reference electrodes and internal reference solu- tion used, and (RT In 10)lF is the Nernst factor, 59.2 mV at 25" C. From this equation, it is evident that a plot of the logarithm of the fluoride activity L'S. the electrode potential should result in a straight line with a slope of 59.2 mV/pF. In most analytical applications, the amount of fluoride de- termined is more usefully expressed in terms of the fluoride concentration, although this type of calibration curve ex- hibits less linearity because of activity coefficient changes. To prevent significant changes in the activity coefficient due to the presence of small amounts of extraneous salts in the system (e.g., by diffusion of KCl from the SCE), all solutions were prepared containing 0.1M K N 0 3 to maintain a relatively constant ionic strength, except where the fluoride concen- tration itself becomes large.

mole of NaFjliter of 0.1M K N 0 3 were used to obtain the calibration, or work- ing, curve (Figure 2) for the modified electrode. The data, from Table I, are plotted in terms of both the activity and

Test solutions containing 10-l to

concentration of fluoride. The fluoride activity in the test solutions is obtained from activity coefficient data, calculated from the Debye-Huckel theory using the values of Kielland (2) , based on the total ionic strength of the solution. The electrode exhibits excellent Nernstian response to the fluoride activity (approximately 59.5 mV/pF), and the calibration curve (Figure 2) is linear from 10-'M to less than 10-4M NaF. The calibration curve for fluoride concentration exhibits comparable linearity over this same range although deviating slightly above 10-2M NaF.

The precision of the emf measurements is indicated by the radius of the open circles in Figure 2 and represents the stand- ard deviation of a single measurement. The potential re- ported for each concentration in Table I is the average of seven observations made over the period of a week. In addition, each of these seven values is actually the average of triplicate measurements on consecutive aliquots of the same stock solution. The precision of the measurements made on the same day was only slightly better than that be- tween days indicating good stability of the fluoride electrode from day to day. The long-term deviation of the electrode was found to be random (bidirectional) and approximately 2 mV/week. Since the ambient temperature was maintained at 25" + 1 ' C, part of the apparent electrode instability could be due to temperature variations. Of even greater signifi- cance on the long-term stability of the emf measurements --

(2) J. Jcielland. J . Am. Cliern. S O ~ . , 59, 1675 (1937).

4 Figure 2. Electrode response to fluoride concentration and activity

1484 ANALYTICAL CHEMISTRY

may well be the inherent instability of the reference elec- trode (SCE) and the variability of the liquid junction poten- tial between the SCE and test solution. In general, the emf at all concentrations exhibited similar shifts so that, in normal use, daily standardization of the electrode at one concentra- tion, preferably within the range to be studied, would permit the use of the same calibration curve,

The precision reported in Table I is calculated from the slope of the calibration curve (Figure 2) at the various fluoride concentrations and the standard deviation in measuring the emf. That is, the precision in measuring the emf determines how precisely the fluoride concentration can be read from the calibration curve. For example, for the 10-IM NaF solu- tion, the slope of the curve is approximately 53 mV/pF, and the resulting concentration error is 4.5 %/mV. Since the standard deviation of a single emf reading at this point is 0.47 mV. the calculated precision for a single determination is 2.1 or 2 pg.

Below lO-'M NaF, the slope of the calibration curve be- comes progressively less, from 46 mV/pF at lO-5M to 10 mV/pF at 10-GM NaF, thereby increasing the error associated with the variability in the emf measurements. At the 10-6M

NaF level, the determination of fluoride by this technique is virtually useless because of the low calibration curve slope and is further impaired by the poor precision of the emf measure- ments. The precision in the determination of 5 X 10-6M NaF is still analytically inadequate (17.2%), and amounts to 0.8 nanogram in a sample containing 5 nanograms of fluoride. At 7 x 10-GM NaF (calculated), the error decreases to 10% and becomes progressively less as the concentration increases to 10-3M fluoride. Above NaF, the precision remains approximately constant at 2 %.

In summary, the modified fluoride electrode exhibits a usable response in the range from 10-'M to 5 X 10-6M fluoride for sample volumes of 50 pl. Although micro- analysis by direct emf measurements using a calibration curve gives acceptable results, improved precision should be ob- tainable using this modified electrode in conjunction with the technique of linear null-point potentiometry (3). Research along these lines is presently in progress.

RECEIVED for review June 16, 1967. Accepted July 14, 1967.

(3) R. A. Durst and J. K. Taylor, ANAL. CHEM., 39, 1374 (1967).

Determination of Aliphatic and Aromatic Acids by Pyrolysis of Their Tetramethylammonium Salts J. J. Bailey Eastern Laborutory, Explosiws Department, E. I . du Pont de Nemours & Co., Gibbstown, N . J .

ORGANIC ACIDS, in the presence of neutral compounds, are normally analyzed by gas chromatography of their methyl esters. Vorbeck, et al. (1) and Kirkland (2) made quanti- tative comparisons of methods of converting carboxylic acids to methyl esters. During the past 2 years, to overcome various methylation problems, we have used the methylation technique originally described by Robb and Westbrook (3) and later by Downing (4) to methylate acid samples. They found that pyrolysis of tetramethylammonium salts of acids takes place instantaneously in the hot injection port of a gas chromatograph to yield trimethylamine and the methyl esters of the acids, which are swept onto the column and separated by gas chromatography. Although Robb and Westbrook stated that this esterification procedure is inapplicable for assay-type analyses, we have applied it successfully to the de- termination of mono- and dibasic aromatic acids when present as major components in mixtures containing nonacidic com- ponents and to similar mixtures containing mono- and dibasic aliphatic acids and esters.

EXPERIMENTAL

An F & M (Scientific Division of Hewlett- Packard) Model 810 gas chromatograph fitted with a thermal conductivity detector was used in this study. The operating parameters were: helium flow rate of 100 ml/minute, de-

Apparatus.

(1) M. L. Vorbeck, L. R. Hattick, F. A. Lee, and C. S. Pederson,

(2) J. J. Kirkland, Zbid., 33, 1520 (1961). (3) E. W. Robb and J. J. Westbrook, Zbid., 35, 1644 (1963). (4) D. T. Downing, Zbid., 39,218 (1967).

ANAL. CHEM., 33, 1512 (1961).

tector temperature of 300' C, injection port (packed loosely with glass wool) maintained between 360" and 380" C, bridge current of 170 mA, and recorder chart speed of 0.5 inchiminute. Columns used were either a 4-foot X 0.25- inch 0.d. column packed with 2 0 z Carbowax 20M on 30-60 mesh Chromosorb W to determine aliphatic acids, or a 6.5- foot x 0.25-inch 0.d. column of 15% FFAP (Wilkens Aerograph) on 90-100 mesh Arakrom ABS to separate aro- matic acids.

The temperature of the FFAP column was maintained at 75" C for 3 minutes after injection of the sample, then pro- grammed at 10" Ciminute to a maximum temperature of 275" C and held at this temperature to allow elution of the high boilers. The temperature of the 20M column was held at 75" C for 5 minutes, then programmed at 6" C/minute and held at 250" C: until all components were eluted.

To ensure high accuracy in the analysis of aromatic acids, internal standards were employed. The internal standards solution was pre- pared by accurately weighing 31 grams of benzyl ether (BE), 102 grams of 2-methylnaphthalene (MEN), and 31 grams of mesitylene (MES) into a 250-ml volumetric flask, then diluting the solution to volume with absolute methanol. The purity of each internal standard was determined by gas chromatog- raphy, retention times of impurity peaks were noted, and appropriate corrections were applied to chromatograms of samples in which internal standard impurity peaks coincided with peaks of sample components. From calculated purities, the correct weight of each internal standard per 2 ml of solution was obtained.

Preparation of a Simulated p-Xylene Air Oxidation Product Containing Aromatic Acids and Nonacidic Components. The following compounds were weighed into a tared 25-ml volu- metric flask: 0.400 gram of terephthalic acid (TPA), 1.400

Preparation of Internal Standard Solution.

VOL. 39, NO. 12, OCTOBER 1967 1485